In order to resist the high temperatures (up to about 1600.degree. C.) occurring in decarburization furnaces used in the steel industry, these furnaces must be lined with fire resistant material, such as dolomite or magnesite. The linings are initially quite thick, about 0.5 meters, but are worn during the decarburization procedure. Before the lining material is diminished below a certain critical thickness, at which thickness there exists the risk that the supporting steel mantle will be burnt through, the furnace must be shut down for relining. Such a relining is expensive, partly because of the relining costs themselves and partly because the furnace is not functioning for a period of time.
It is therefore apparent that a need exists for a method for measuring the thickness of the lining in order to have it under safe control when the furnace is operating. Such a measuring method should meet with the two following requirements:
(1) Short measuring time and a quick estimation of the results of the measurement, so that steps can be taken for relining, in good time; and
(2) Good accuracy of the measurement, which means in practice that the thickness of the lining in question shall be known with an accuracy of about 5 mm. It is also essential to find out the shape of the profile of the lining both in the axial direction and along the circumference inside the furnace. This is because it will then be possible, by controlling the process, to obtain a more even wear of the lining which is evidently of economical advantage.
No method used so far has met with both the above demands. A visual observation, for instance, is quick but gives bad accuracy. By stereophotogrammetry it is possible to measure to an accuracy of perhaps 30-40 mm, but the method is slow, and the result is not obtained until after one or more days. Therefore, this method is not suitable for operation control.
An isotope method is available which operates by analyzing samples of the charge to decide how much has been dissolved of a wire, radially inserted in the lining and having good radioactive properties. The method gives high accuracy, but like the foregoing method, it has a long evaluation time and is more suitable for the calibration of other measuring methods.
A quick and safe method would be to measure the distance from a point outside the furnace to one or more points on the furnace lining. This is quite possible if the furnace is allowed to cool, whereby common measuring methods may be used. However, such a cooling of the furnace is very time-consuming and thereby expensive and the reheating requires much energy.
It is therefore an object of the present invention to provide apparatus by the use of which it is possible to make a dimension measurement which is very accurate and meets with the abovementioned demands. It is another object of the present invention to provide such apparatus which may be placed outside the furnace and allow measurements to one or more points on the furnace lining, while the furnace is hot.